1. Field of the Invention
The present invention relates to an optical transmission apparatus, and, more particularly, to an optical transmission apparatus which performs optical transmission over an optical transmission passage without electric regeneration.
2. Description of the Related Art
There have been active studies and developments on optical communication techniques. Such active studies and developments on optical communication have been made due to the advantages of optical signal transmission over electric signal transmission in the transmission speed, the interference between signals, and so on. Under this circumstance, since an optical transmitter which transmits optical signals of a stable wavelength even under the modulation, ultra-high speed LSIs, optical amplifiers, etc., are developed, ultra-high speed optical fiber transmission without electric regeneration over several hundred kilometers at the rate of 10 Gb/s is going to be accomplished.
To transmit light signals modulated at the rate of over 1 Gb/s over a long distance without electric regeneration, an optical transmitter or an intermediate repeater sends light power of over several mW to a transmission optical fiber in order to extend the distance between repeaters or reduce the influence of spontaneous emission noise in optical amplifiers.
In the case of such high power transmission, the spectrum will be expanded during transmission by the nonlinearity of the optical fiber (Kerr effect). The Kerr effect is a phenomenon that the refractive index varies in proportion to the square of the electric field, and the proportional coefficient or the nonlinear refractive index n.sub.2 has a value of about 3.2.times.10.sup.-6 cm.sup.2 /W in the optical fibers. Therefore, a strong and short light pulse after fiber transmission has a long wavelength at the rising and a short wavelength at the falling. In other words, self phase modulation occurs. In the optical fibers, therefore, a light pulse with a wavelength indicating anomalous dispersion is compressed while a light pulse with a normal dispersion wavelength is expanded.
The received eye pattern is the widest when proper pulse compression is applied. To eliminate the influence of the Kerr effect and keep the bit error rate low, therefore, the wavelength dispersion should be set within a predetermined range in the vicinity of the optimum value. In particular, to accomplish long-distance transmission using optical amplifiers without electric regeneration, the wavelength dispersion allowance to achieve a low bit error rate becomes narrower.
As the dispersion characteristic and nonlinearity of optical fibers vary slightly depending on a change in the environmental conditions or on the maintenance, it is generally difficult to control the characteristic of the optical fibers once laid out. Normally, the oscillation wavelength is controlled so as to provide a predetermined wavelength dispersion by changing the heat sink temperature of a laser as the light source of an optical transmitter.
But, the temperature-based control has a large time delay (several seconds), so that continuous and stable automatic control is difficult to achieve. In an optical fiber transmission system constituted of a plurality of sections separated by optical repeaters, the optimum wavelength dispersion value varies slightly from one section to another. The optimum control section by section cannot be accomplished through wavelength control. Further, since the optimum wavelength range is narrow for the laid fibers, the number of channels that can be wavelength-division-multiplexed is limited.
As described above, the conventional optical transmission apparatus has a shortcoming such that it is difficult to accomplish continuous and stable automatic control so as to maintain the optimum eye pattern with respect to a change in the characteristic of optical fibers.